Hydraulic system

ABSTRACT

A method includes providing a hydraulic pump with an outlet, a main orifice having an inlet communicating with the pump outlet, and a main orifice outlet. Further provided are a flow orifice, an amplification orifice, and an amplification orifice outlet, means for generating an output signal representative of a fluid pressure, and a pump controller for controlling the hydraulic pump in response to the output signal. The flow orifice and the amplification orifices each define an area, with these areas defining a ratio, and with the main orifice and the ratio being variable. The method includes operating the system in a first mode so as to define a first mode ratio and operating the system in a second mode so as to define a second mode ratio, so that the ratio is controlled differently in the first mode and the second mode.

FIELD OF THE INVENTION

The present invention relates to a hydraulic system, in particular ahydraulic system used on working machines.

BACKGROUND OF THE INVENTION

A typical working machine may include a hydraulic system having ahydraulic pump and one or more hydraulically operated services (such asactuators) coupled to the hydraulic pump. One or more control valves areused to control the supply of hydraulic fluid from the hydraulic pump tothe or each actuator. Thus, an operator may use a control interface tocontrol operation of the one or more control valves to cause actuationof one or more of the actuators.

The actuators may be coupled to parts of the working machine. Forexample, the actuation of an actuator may cause movement of a workingarm of the working machine.

Known load sensing hydraulic systems aim to keep a constant flow for agiven position of a control valve, in particular for a given position ofa spool of a directional control spool valve. This is done bymaintaining a constant pressure difference known as (valve) marginpressure, across the orifice made by the spool.

The associated pump has a control valve that automatically keeps thepump pressure and flow at a level needed to fulfill the system load andflow needs. When none of the hydraulic circuits are being used the pumpwill be operating at a “stand by” pressure, typically be somewhere therange 20-30 bar. When a hydraulic service is being used a signalrepresentative of the pressure demanded at the service is sent to thepump control valve which then controls the pump to operate at therequired pump pressure and flow. The pump outlet pressure will typicallybe somewhere in the range 20-30 bar above the pressure at the serviceand the difference between the pump supply pressure and the servicepressure is called the pump margin pressure. In known load sensingsystems the pump margin pressure and the valve margin pressures areusually identical (ignoring any line losses).

On a variable displacement hydraulic pump the margin pressure is set bya load sense controller bias spring. On a fixed displacement hydraulicpump the margin pressure is set by a bypass regulator valve bias spring.It is not possible to adjust the spring pressure of the load sensecontroller bias spring or the bypass regulator valve bias spring whilstthe machine is in operation i.e. whilst the machine is being used and assuch the spring pressure is fixed whilst the machine is being used.

Typically a service will react more quickly with a higher valve marginpressure than with a lower valve margin pressure. A higher marginpressure can be advantageous in some circumstances, for example when aloading shovel is being used to load loose material, such as earth.However, under other circumstances a higher margin pressure can bedisadvantageous. For example, where an operator needs to carefullycontrol the position of a service. With a higher margin pressure theservice can react too quickly to operator inputs making precise controlof the service difficult. Additionally, higher margin pressures for lowflow requirements represent an unnecessary energy loss. An example wherecareful control of a service is required would be operative of a loadingshovel during a grading operation (i.e. an operation where a groundsurface is leveled off or graded by taking a thin skim off the ground).In such prior art systems the margin pressure has to be set at acompromise.

Other prior art has attempted to vary the margin pressure at the pump,and so affect the valve margin pressure to offer additional levels offlow control, but they suffer from a lack of range (i.e. they start atthe pre-set level and allow only a gradual reduction in margin pressure,towards zero, by offsetting the spring load by electrical solenoid orhydraulic pilot pressure means.

Operation of the hydraulic pump consumes fuel (the hydraulic pump istypically coupled to an engine of the working machine which drives thehydraulic pump). Therefore, there is a desire to operate the hydraulicpump and valves as efficiently as possible.

There is also a desire to provide hydraulic fluid quickly with minimaltransmission losses when it is required to avoid any significant lagbetween, for example, an operator using a control interface to causeactuation of an actuator and the actuator actuating as a result.

The distribution of hydraulic fluid between a plurality of controlvalves and associated actuators is difficult as, for example, thetermination of the operation of one actuator can have a significantimpact on the pressure of the hydraulic fluid in the hydraulic systemand, hence, the operation of the other actuators. There is a desire toreduce the unwanted impact of changes in the demand for hydraulic fluidon the operation of actuators of a hydraulic system.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome one or more problemsassociated with the prior art.

Thus, according to one aspect of the present invention there is provideda method of operating a hydraulic system including providing a hydraulicpump having a pump outlet, a main orifice having a main orifice inlet influid communication with the pump outlet and a main orifice outlet forsupplying pressurized fluid to a service, a flow orifice having a floworifice inlet for sensing a pressure representative of a pressure at thepump outlet and a flow orifice outlet, an amplification orifice havingan amplification orifice inlet in fluid communication with the floworifice outlet and an amplification orifice outlet for sensing apressure representative of a pressure at a service, means for generatingan output signal representative of a fluid pressure between the floworifice outlet and the amplification orifice inlet, and a pumpcontroller for controlling the hydraulic pump in response to the outputsignal, the flow orifice defining a flow orifice cross section area andthe amplification orifice defining an amplification orifice crosssection area, the flow orifice cross section area and the amplificationorifice cross section area defining a ratio, in which the main orificeis variable and the ratio is variable, the method further comprising thesteps of operating the system in a first mode so as to define a firstmode ratio regime and operating the system in a second mode so as todefine a second mode ratio regime, so that the first mode ratio regimeis different to the second mode ratio regime.

The method may include operating the system in the first mode when thefirst mode ratio regime is to have a fixed ratio.

The method may include the first mode ratio regime which fixes the ratioat greater than 1 or less than 1 or equal to 1.

The method may include operating the system in the first mode when thefirst mode ratio regime is to have a variable ratio.

The method may include the variable ratio including a ratio of greaterthan 1 and/or including a ratio of less than 1 and/or including a ratioequal to 1.

The method may include the variable ratio excluding a ratio of greaterthan 1 or wherein the variable ratio excluding a ratio of less than 1.

The method may include operating the system in the second mode when thesecond mode ratio regime is to have a fixed ratio.

The method may include the second mode ratio being greater than 1 orless than 1, or equal to 1.

The method may include operating the system in the second mode when thesecond mode ratio regime is to have a variable ratio.

The method may include having a variable ratio which includes a ratio ofgreater than 1 and/or includes a ratio of less than 1 and/or includes aratio equal to 1.

The method may include having a variable ratio which excludes a ratio ofgreater than 1 or wherein the variable ratio excludes a ratio of lessthan 1.

The method may further comprise the steps of operating the system in athird mode so as to define a third mode ratio regime so that the ratiois controlled differently in the first mode and the second mode and thethird mode wherein when operating the system in the third mode the thirdmode ratio regime is to have a fixed ratio.

The method may include the third mode ratio being greater than 1 or lessthan 1, or equal to 1.

The method may further comprise the steps of operating the system in athird mode so as to define a third mode ratio regime so that the ratiois controlled differently in the first mode and the second mode and thethird mode wherein when operating the system in the third mode the thirdmode ratio regime is to have a variable ratio.

The method may include the variable ratio including a ratio of greaterthan 1 and/or including a ratio of less than 1 and/or including a ratioequal to 1.

The method may include the variable ratio excluding a ratio of greaterthan 1 or wherein the variable ratio excludes a ratio of less than 1.

The method may include the amplification orifice outlet is in fluidcommunication with a service.

The method may include the flow orifice being variable.

The method may include the amplification orifice being variable.

The method may include the flow orifice being fixed.

The method may include the amplification orifice being fixed.

The method may include the main orifice being variable only between afirst position and a second position.

The method may include the flow orifice being variable only between afirst position and a second position.

The method may include the amplification orifice being variable onlybetween a first position and a second position.

The method may include the first position being a closed position.

The method may include the main orifice being variable between a firstposition, a second position and a third position.

The method may include the flow orifice being variable between a firstposition, a second position and a third position.

The method may include the amplification orifice being variable betweena first position, a second position and a third position.

The method may include the first position being a closed position.

The method may include the main orifice being continuously variable.

The method may include the flow orifice being continuously variable.

The method may include the amplification orifice being continuouslyvariable.

The method may include the hydraulic pump being a variable displacementhydraulic pump having a pump margin pressure and the controller isconfigured to vary a margin pressure of the main orifice relative to thepump margin pressure in response to the output signal.

The method may include the hydraulic pump is a fixed displacementhydraulic pump, having a pump margin pressure defined by a bypassregulator valve and the controller is configured to vary a marginpressure of the main orifice relative to the pump margin pressure inresponse to the output signal.

The method may include the controller being configured to increase themargin pressure of the main orifice relative to the pump margin pressurein response to the output signal.

According to a further aspect of the present invention there is provideda hydraulic system including a hydraulic pump having a pump outlet, amain orifice having a main orifice inlet in fluid communication with thepump outlet and a main orifice outlet for supplying pressurized fluid toa service, a flow orifice having a flow orifice inlet for sensing apressure representative of a pressure at the pump outlet and a floworifice outlet, an amplification orifice having an amplification orificeinlet in fluid communication with the flow orifice outlet and anamplification orifice outlet for sensing a pressure representative of apressure at a service, means for generating an output signalrepresentative of a fluid pressure between the flow orifice outlet andthe amplification orifice inlet, and a pump controller for controllingthe hydraulic pump in response to the output signal, the flow orificedefining a flow orifice cross section area and the amplification orificedefining an amplification orifice cross section area, the flow orificecross section area and the amplification orifice cross section areadefining a ratio, in which the main orifice is variable and the ratio isvariable, the main orifice defining a main orifice margin pressure, thesystem being configured to vary the main orifice margin pressure byvarying the ratio.

The hydraulic system may include the pump defining a pump marginpressure and the system being configured to vary the main orifice marginpressure relative to the pump margin pressure.

The hydraulic system may include the main orifice margin pressure beinggreater than the pump margin pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of embodiments of the present invention as described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 shows a working machine incorporating a hydraulic systemaccording to the present invention;

FIG. 2 shows an embodiment of a hydraulic system according to thepresent invention,

FIG. 3 shows a hydraulic system according to the present invention, and

FIG. 4 shows a hydraulic system according to the present invention.

With reference to FIG. 1, there is shown a working machine 10.

DETAILED DESCRIPTION

The working machine 10 may be a machine as generally depicted in FIG. 1;however, it will be appreciated that embodiments of the presentinvention may be used in relation to other types of working machine andthe machine depicted in FIG. 1 is merely shown by way of example.

The working machine 10 includes a main body 12 which may include a cab14 mounted thereon. The main body 12 of the machine carries an engine16. The main body 12 may include a ground engaging arrangement 18. Theground engaging arrangement may comprise, for example, a plurality ofwheels mounted on a plurality of axles and/or may comprise one or moreendless tracks. The ground engaging arrangement 18 is coupled to theengine 16 which is configured to drive the ground engaging arrangement18 with respect to a ground surface to cause movement of the main bodyacross the ground surface.

The working machine 10 may include one or more working arms 20 on whichmay be mounted respective working tools or implements 22. The workingmachine may include two working arms 20, for example, as depicted inFIG. 1. The or each working arm 20 may comprise a plurality of armsections coupled to each other—for example, with the distal end of onearm section coupled in a pivotable configuration to a proximal end ofanother arm section. The or each working arm 20, or a part thereof, maybe configured for movement with respect to the main body 12 of theworking machine 10. A working arm 20 may comprise a boom coupled to adipper arm.

Movement of the or each working arm, or part thereof, may be driven by arespective service such as an actuator 24 which may be a hydraulicactuator 24.

Other services may be provided to drive movement of other components ofthe working machine 10.

A hydraulic system 30 (see FIG. 2) is provided to control and drive ahydraulic service of the working machine such as one or more of the oreach actuator 24 of the working machine 10.

Hydraulic system 30 includes a hydraulic pump 32 driven by the engineand having a pump outlet 60 and a pump inlet 65 in fluid communicationwith a source of hydraulic fluid 34 in the form of a tank. The hydraulicpump supplies pressurized fluid to a hydraulic service (in this case anactuator 24) primarily via main orifice 43 as described below.

The hydraulic system has a main fluid path 61 and a secondary fluid path62. The main fluid path is in fluid communication with the pump outlet60 and includes a main orifice 43. The secondary fluid path 62 is inparallel with the main orifice 43. The secondary fluid path includes aflow (or pilot) orifice 44 and an amplification orifice 45 in serieswith the flow orifice. Means 63 is capable of generating an outputsignal representative of the fluid pressure in the secondary fluid path62 between the flow orifice 44 and the amplification orifice 45 in thiscase means 63 is a port connected to pressure sensing line 66. A pumpcontroller 64 is capable of controlling the pump flow in response to theoutput signal. The main orifice 43 is variable and at least one of theflow orifice 44 and amplification orifice 45 is a variable orifice.

Pump 32 may be a variable displacement pump or it may be a fixeddisplacement pump.

When the pump 32 is a variable displacement pump the pump controller 64may act to vary the pump flow. For example where pump 32 is a variabledisplacement swash plate pump then the pump controller 64 may act tovary the angle of the swash plate, thereby varying the pump flow.

Where the pump 32 is a fixed displacement pump the fixed displacementpump may include a bypass valve through which excess fluid flow can passon route to a reservoir (or tank) of hydraulic fluid (such as source ofhydraulic fluid 34). The pump controller 64 may vary the relief pressureof the bypass valve, thereby varying the amount of excess fluid flowthat passes to tank.

As mentioned above the main orifice 43 is a variable orifice, which maybe variable between just two orifice areas, or it may be variablebetween two or more discreet orifice areas, or it may be continuouslyvariable over a range of orifice areas. The smallest orifice area of themain orifice may be zero (i.e. the main orifice may be closable) or thesmallest orifice area may be a non zero area (i.e. the main orifice maynot be closable). The main orifice 43 may be varied manually orelectrically, or via a pilot pressure.

The flow orifice (or pilot orifice) 44 may be a fixed orifice or may bea variable orifice. When the flow orifice 44 is a variable orifice, itmay be variable between just two orifice areas, or it may be variablebetween two or more discreet orifice areas, or it may be continuouslyvariable over a range of orifice areas. When the flow orifice 44 is avariable orifice the smallest orifice area may be zero (i.e. the floworifice may be closable) or the smallest orifice area may be a non zeroarea (i.e. the flow orifice may not be closable). Where flow orifice 44is variable it may be varied manually, or electrically, or via a pilotpressure.

The amplification orifice 45 may be a fixed orifice or may be a variableorifice. When the amplification orifice 45 is a variable orifice, it maybe variable between just two orifice areas, or it may be variablebetween two or more discreet orifice areas, or it may be continuouslyvariable over a range of orifice areas. When the amplification orifice45 is a variable orifice the smallest orifice area may be a non zeroarea (i.e. the amplification orifice may not be closable). Whereamplification orifice 45 is variable it may be varied manually, orelectrically, or via a pilot pressure.

Where two or more of main orifice 43, flow orifice 44 and amplificationorifice 45 are variable, they may be varied together or they may bevaried independently.

The means 63 for generating an output signal representative of the fluidpressure in the secondary fluid path between the flow orifice andamplification orifice may be a tapping generating a pilot pressuresignal or may be a pressure sensor that generates an electrical signal.

Table 1 below shows three options with regard to the main orifice, floworifice and amplification orifice being either variable or fixed.

TABLE 1 Flow Orifice Amplification Options Main Orifice (pilot orifice)Orifice 1 V V V 2 V F V 3 V V F V = variable F = fixed

Consideration of FIG. 2 shows that:

-   hydraulic lines 50, 61A and 62A will all be at the same hydraulic    pressure namely the pump outlet pressure,-   hydraulic lines 61B, 62C and 51 will all be at the same hydraulic    pressure namely the service pressure, or load sense pressure, i.e.    the pressure sensed at the load (or service).

When the service is being operated the pump will be pumping hydraulicfluid along line 50, and line 61A, through the main orifice 43 alongline 61B and line 51 to the service 24. Because there will be a pressuredrop across the main orifice as the hydraulic fluid passes through theorifice then the pump outlet pressure will be higher than the servicepressure, the difference between the pump outlet pressure and servicepressure being the margin pressure.

The pressure in the line 62B between the flow orifice will be less thanthe pump outlet pressure but will be greater than the service pressure.

The actual value will depend upon the relative cross sectional areas ofthe flow and amplification orifices.

Thus if the flow orifice cross section area is larger than theamplification orifice cross section area then the pressure in line 62Bwill be nearer the pump outlet pressure than the service pressure.

Conversely if the cross section area of the flow orifice is smaller thanthe cross section area of the amplification orifice then the pressure inline 62B will be nearer the service pressure than the pump outletpressure.

Thus by varying the ratio of the cross section area of the flow orificeand amplification orifice the pressure in the line 62B (the intermediatepressure) between the flow orifice and amp orifice can be varied andwill be less than the pump outlet pressure but more than the servicepressure. This intermediate pressure can then be communicated to thepump via line 66 so that the pump swash setting is controlled to matchthe flow demands.

The present invention allows the system to be operated in a first modeand in a second mode such that the ratio is controlled differently inthe first mode and the second mode.

As mentioned above, in prior art load sensing systems, the pump marginpressure and valve margin pressure are usually identical (ignoring anyline losses). The present invention allows the valve (or orifice) marginpressure to be varied relative to the pump margin pressure. This is doneby varying the flow orifice area to amplification orifice area ratio.Thus the valve margin pressure equals the pump margin pressure plus (thearea ratio squared times the pump margin pressure), in other words:

valve  margin  pressure = pump  margin  pressure + ((area  ratio)² × pump  margin  pressure)or${{valve}\mspace{14mu} {margin}\mspace{14mu} {pressure}} = {{pump}\mspace{14mu} {margin}\mspace{14mu} {pressure} \times \left( {1 + \frac{{flow}\mspace{14mu} {orifice}\mspace{14mu} {area}^{2}}{{amplification}\mspace{14mu} {orifice}^{2}}} \right)}$

Thus the first mode of operation defines a first mode ratio regime andthe second mode defines a second mode ratio regime which is differentfrom the first mode ratio regime.

In one embodiment in one mode the cross section area of the flow orificemay be 3 mm2 and the cross section area of the amplification orifice maybe set to 4 mm2, giving a ratio of 3:4 i.e. 0.75. This ratio remainsconstant, setting a fixed valve margin pressure (in this case equivalentto the pump margin×(1+0.752), different to the pump margin pressureproportional to the ratio. When operating the machine in this mode withorifice 43 varying from zero to some max value flow control of theservice is established. In this mode the valve orifice 43 will have arelatively low margin pressure of about 1.56 times the pump marginpressure across it and hence this mode is suitable for precision work,such as grading. In this mode the ratio regime is to have a fixed ratioi.e. when operating in this mode the ratio does not change, i.e. theratio remains constant.

In another mode, the cross section area of the flow orifice may be 3 mm2and the cross section area of the amplification orifice may be set to 1mm2, giving a ratio of 3:1 i.e. 3. This ratio remains constant whenoperating the machine in this mode. In this mode the valve orifice 43will have a relatively high margin pressure (about 10 times the pumpmargin) across it and hence this mode is suitable for fast work such asloading. In this mode the ratio regime is to have a fixed ratio i.e.when operating in this mode the ratio does not change, i.e. the ratioremains constant.

In another mode, the cross section area of the flow orifice may be 3 mm2and the cross section area of the amplification orifice may vary between4 mm2 at a relatively low displacement of an associated spool and 1.5mm2 at a relatively high displacement of an associated spool. Underthese circumstances the ratio will vary between 3:4, i.e. 0.75 atrelatively low displacements and 3:1, i.e. 3 at relatively highdisplacements. Under these circumstances, the valve orifice 43 will havea relatively low margin (of about 1.56 times the pump margin pressureacross it) at low spool displacement and will have a relatively highmargin (of about 10 times the pump margin across it) at high spooldisplacement and hence this mode is suitable for precision work such asgrading at low spool displacement and is suitable for fast work, such asloading at high spool displacement spool. The spool may typically be acontrol spool for controlling the service (such as an actuator). In thismode the ratio regime is to have a variable ratio i.e. when operating inthis mode the ratio changes i.e. the ratio does not remain constant.

In the above example it will be appreciated for all three modes ofoperation the flow orifice cross section area is 3 mm2, i.e. the floworifice cross section area is fixed. By controlling the amplificationorifice in three different ways (set at 4 mm2, set at 1 mm2, variablebetween 1 mm2 and 4 mm2) then it is possible to control the ratio inthree different ways (the ratio fixed at 0.75, the ratio fixed at 3, theratio variable between 0.75 and 3) i.e. it is possible to have threedifferent ratio regimes by being able to control the ratio in threedifferent ways (i.e. by having three different ratio regimes) then thepressure at the means 63 and in hydraulic lines 66 can be controlled inthree different ways allowing the valve margin pressure across orifice43 to be controlled in three different ways.

In the example above the main orifice is variable, the amplificationorifice is variable and the flow orifice is fixed and this equates tooption 2 in table 1 above. In an alternative embodiment theamplification orifice could be fixed and the flow orifice could bevariable as in option 3 in table 1 above. In one mode the cross sectionarea of the flow orifice may remain constant during operation in thismode. In a different mode of operation the cross section area of theflow orifice may remain constant at a different cross section area. Inan alternative mode the cross section area of the flow orifice may varydependent upon a characteristic of machine (for example dependent uponthe position of associated spool). By controlling the cross section areaof the flow orifice in different ways allows control of the ratio ofcross section area of the flow orifice and amplification orifice indifferent ways and this in turn allows the valve margin pressure acrossorifice 43 to be controlled in different ways.

In option 1 in table 1 above, both the flow orifice and theamplification orifice are variable. Under these circumstances in oneembodiment in one mode, the cross section area of the flow orifice maybe set to 2 mm2 and the cross section area of the amplification orificemay be set to 4 mm2 giving a ratio of 2:4 or 0.5. This ratio remainsconstant when operating the machine in this mode. In this mode the valvewill have a relatively low pressure margin and hence this may besuitable for precision work, such as grading.

In another mode the cross section area of the flow orifice may be set to4 mm2 and the cross section area of the amplification orifice may be setto 2 mm2 giving a ratio of 4:2 i.e. 2. This ratio remains constant whenoperating the machine in this mode. In this mode the valve will have arelatively high margin and hence this mode is suitable for fast worksuch as loading.

In another mode the cross section area of either the amplificationorifice or the flow orifice or both orifices may vary depending upon acharacteristic of the machine, for example depending upon the positionof an associated spool. In one mode the cross section area of theamplification orifice may vary whilst cross section area of the floworifice remains constant. In another mode the cross section area of theflow orifice may be variable whilst the cross section area of theamplification orifice remains constant. In another mode the crosssection area of the amplification orifice may vary and the cross sectionarea of the flow orifice may vary. As will be appreciated, by arrangingfor the cross section area of the flow orifice to be variable and thecross section area of the amplification orifice to be variable allowsthe system to be operated in a first mode and in a second mode such thatthe ratio is controlled differently in the first mode and the secondmode.

With reference to FIG. 3 there is shown a further hydraulic system 230according to the present invention with components that fulfill the samefunction as hydraulic system 30 being labeled 200 greater. Pump 232 is afixed displacement pump. A bypass regulator 270 allows excess fluid flowto pass to tank and is controlled via a port 263 between the floworifice 244 and amplification orifice 245 and pressure sensing line 266.The flow orifice and amplification orifice are in series with theamplification orifice being downstream of the flow orifice.

In this case there are two main orifices 243A and 243B. The main orifice243A is defined by orifice 243A1 and orifice 243A2 of control valve 271.Orifice 243B is defined by orifice 243B1 and 243B2 contained withincontrol valve 272.

Load sensing copy valve 277 is provided. Copy valves are known per seand act to replicate the pressure either side of the valve. Thus, thepressure at C is replicated by the copy valve such that the pressure atC′ is the same as the pressure at C.

Control valve 271 controls service 273 and control valve 272 controlsservice 274. A compensator 275 is associated with control valve 271 anda compensator 276 is associated with control valve 272. Compensators perse are known and act to reduce pressure being supplied to the associatedservice under certain conditions. Thus, for the purpose of explanation,it is assumed that service 273 requires a higher pressure than service274. Accordingly, the spool of compensator 275 will be positioned asshown in FIG. 3 and when the control valve 271 is operated, thenhydraulic fluid will pass through the control valve 271, through thecompensator 275 (without any significant loss in pressure) throughcontrol valve 271 to service 273.

However, because, in this example, service 274 operates at a lowerpressure, the spool of compensator 276 will be positioned towards theleft when viewing FIG. 3 i.e. the spool will move to the middle positionshown and may move nearly to the fully closed position (the right handbox symbol) since the pressure at D in the hydraulic circuit will beless than the pressure at A and therefore the pressure at C′ will causethe spool of the compensator to move left when viewing FIG. 3. This willresult in fluid flowing through compensator 276 dropping the pressurebetween A and D.

Hydraulic system 330 includes a known drain regulator 280 which isarranged to drain trapped pressure when no service spool is actuatedthus allowing pump pressure to fall back to a low standby value.

Relief valves 281 and 282 are provided to protect service 274. Reliefvalves 283 and 284 are provided to protect service 273. Relief valve 285acts to limit the pressure in line 266 but can be temporarily overriddenby a “boost” valve 286, which, when actuated by the operator results ina pressure drop across the valve thereby enabling the pressure in line266 to increase above the relief valve pressure setting of relief valve285. This “boost” valve is used in circumstances where extra systempressure is temporarily required, for example, during a diggingoperation.

Consideration of FIG. 3 shows the following:

The main orifice 243A and 243B are in fluid communication with the pumpoutlet. Each main orifice 243A and 243B supplies pressurized fluid toits associated service 273 and 274. The flow orifice 244 has a floworifice inlet which senses a pressure representative of the pressure ofthe pump outlet, i.e. in this case it senses the pressure of the pumpoutlet. The flow orifice has an outlet. The amplification orifice 245has an inlet in fluid communication with the flow orifice outlet. Theamplification orifice outlet senses a pressure, C′ representative of apressure C at a service (in the above example, the pressure at service273). Means in the form of port 263 generate an output signalrepresentative of the fluid pressure between the flow orifice outlet andthe amplification orifice inlet. A pump controller (in this case bypassregulator 270) controls the hydraulic pump in response to the outputsignal. The flow orifice defines a flow orifice cross section area inthis case a fixed cross section area. The amplification orifice definesan amplification orifice cross section area, in this case a variablecross section area. The flow orifice cross section area and theamplification cross section area define a ratio. The main orifice 243A,243B is variable. The ratio is variable (by virtue of varying theamplification orifice 245). The hydraulic system 30 can be operated in afirst mode and in a second mode such that the ratio is controlleddifferently in the first mode and the second mode. The ratio iscontrolled differently by virtue of controlling the cross section areaof the amplification orifice differently in the first and second modes.In one mode, the amplification orifice can be controlled by maintainingthe amplification orifice cross section area at a specified value. Inthe second mode of operation the amplification orifice may be maintainedat a specified value different from the value when operating in firstmode. In another mode of operation the cross section area of theamplification orifice may be variable.

In further embodiments in addition to the amplification orifice beingvariable the flow orifice may be variable.

In further embodiments the amplification orifice may be fixed and theflow orifice may be variable.

When the flow orifice is variable it may operate in a first mode wherethe cross section area of flow orifice is fixed. The system may operatein a second mode where the cross section area of the flow orifice isfixed at a different value to when operating the first mode. The systemmay operate in a mode where the cross section area of the flow orificeis variable.

The bypass regulator 270 includes a spring 290 which sets a basic pumppressure margin. In the present invention this basic setting may berelatively low, for example 4 bar. The pressure in line 266 acts toassist spring 290 thereby increasing the pump pressure i.e. the pumppressure will always be 4 bar higher than the pressure in line 266.Because the ratio of cross section areas between the flow orifice andamplification orifice can be varied depending upon which mode ofoperation this system is being operated under, then the pressure in line266 varies depending upon the mode of operation in which the system isoperating and hence the valve orifice 243A1 and 243B1 margin pressurecan be varied depending upon the mode of operation in which the systemis being operated.

With reference to FIG. 4 there is shown a further hydraulic system 330with components that fulfill the same function as hydraulic system 230being labeled 100 greater.

The only difference between hydraulic system 230 and hydraulic system330 is that hydraulic system 330 includes a variable pump withassociated hydraulic circuitry whereas hydraulic system 230 has a fixedpump with associated hydraulic circuitry. Thus, pump 332 is a variabledisplacement pump. In this case pump 332 has a swash plate 394, theangle of which is controlled by swash plate controller 395. The basicpump margin pressure is set by spring 390 and the spring pressure ofspring 390 will be supplemented by the pressure in line 366 to increasethe pump pressure as required. The hydraulic system 330 can operate inthe first mode where the ratio of the cross section area of the floworifice and amplification orifice is controlled in a first manner andcan operate in a second mode where the ratio of the flow control orificecross section area and amplification orifice cross section area iscontrolled in a second manner.

In further embodiments in addition to the amplification orifice beingvariable the flow orifice may be variable.

In further embodiments the amplification orifice may be fixed and theflow orifice may be variable.

When the flow orifice is variable it may operate in a first mode wherethe cross section area of flow orifice is fixed. The system may operatein a second mode where the cross section area of the flow orifice isfixed at a different value to when operating the first mode. The systemmay operate in a mode where the cross section area of the flow orificeis variable.

As mentioned above, the outlet from the amplification orifice 45 sensesthe pressure representative of a pressure at service 24. In this casethe outlet of amplification orifice 45 senses the pressure by being influid communication with the service.

These can be contrasted with the outlet from amplification orifice 245and 345, which, whilst also sensing a pressure representative of apressure at a service (in the example the pressure representative ofpressure at the respective service 273, 373), the outlet from theamplification orifice 245 and 345 is not in fluid communication withrespective service 273, 373.

As described above one ratio regime is to keep the ratio (or ratiovalue) fixed. In the example above the ratio value was 0.75 and thisratio value remained constant when operating the system in this mode. Asdescribed above, another, different ratio regime is to keep the ratio(or ratio value) fixed at a different value, in the example case theratio value was 3 and this ratio value remained constant when operatingthe system in this mode. Thus, it is possible to have two differentratio regimes even though the value of the ratio does not change whenoperating under each of the regimes. The regimes are different becausethe value of the ratio when operating under each regime is different.

As described above, one ratio regime is to vary the ratio (or vary theratio value). The example above the ratio value was varied between 0.86and 2. As will be appreciated, another, different ratio regime would beto vary the ratio differently, for example the ratio could vary between0.5 and 2 to provide a different ratio regime, the ratio could varybetween 0.68 and 3 to provide a different ratio regime. Where the ratioregime has a variable ratio value, the ratio value may vary dependentupon a first machine characteristic in a first ratio regime and may varydependent upon a second machine characteristic in a second ratio regime.

Clearly if one ratio regime has a fixed ratio and another ratio regimehas a variable ratio, then these two ratio regimes will necessarily bedifferent regimes.

1. A method of operating a hydraulic system including providing ahydraulic pump having a pump outlet, a main orifice having a mainorifice inlet in fluid communication with the pump outlet and a mainorifice outlet for supplying pressurized fluid to a service, a floworifice having a flow orifice inlet for sensing a pressurerepresentative of a pressure at the pump outlet and a flow orificeoutlet, an amplification orifice having an amplification orifice inletin fluid communication with the flow orifice outlet and an amplificationorifice outlet for sensing a pressure representative of a pressure at aservice, means for generating an output signal representative of a fluidpressure between the flow orifice outlet and the amplification orificeinlet, and a pump controller for controlling the hydraulic pump inresponse to the output signal, the flow orifice defining a flow orificecross section area and the amplification orifice defining anamplification orifice cross section area, the flow orifice cross sectionarea and the amplification orifice cross section area defining a ratio,in which the main orifice is variable and the ratio is variable, themethod further comprising the steps of operating the system in a firstmode so as to define a first mode ratio regime and operating the systemin a second mode so as to define a second mode ratio regime, so that thefirst mode ratio regime is different to the second mode ratio regime. 2.A method as defined in claim 1 wherein when operating the system in thefirst mode the first mode ratio regime is to have a fixed ratio.
 3. Amethod as defined in claim 2 wherein the first mode ratio regime fixesthe ratio at one of greater than 1 and less than 1 and equal to
 1. 4. Amethod as defined in claim 1 wherein when operating the system in thefirst mode the first mode ratio regime is to have a variable ratio.
 5. Amethod as defined in claim 4 wherein the variable ratio excludes a ratioof one of greater than 1 and less than
 1. 6. A method as defined inclaim 1 wherein when operating the system in the second mode the secondmode ratio regime is to have a fixed ratio.
 7. A method as defined inclaim 1 wherein when operating the system in the second mode the secondmode ratio regime is to have a variable ratio.
 8. A method as defined inclaim 1, the method further comprising the steps of operating the systemin a third mode so as to define a third mode ratio regime so that theratio is controlled differently in the first mode and the second modeand the third mode wherein when operating the system in the third modethe third mode ratio regime is to have a fixed ratio.
 9. A method asdefined in claim 1, the method further comprising the steps of operatingthe system in a third mode so as to define a third mode ratio regime sothat the ratio is controlled differently in the first mode and thesecond mode and the third mode wherein when operating the system in thethird mode the third mode ratio regime is to have a variable ratio. 10.A method as defined in claim 1 wherein the amplification orifice outletis in fluid communication with a service.
 11. A method as defined inclaim 1 in which the flow orifice is variable.
 12. A method as definedin claim 11 in which the amplification orifice is variable.
 13. A methodas defined in claim 1 in which the flow orifice is fixed.
 14. A methodas defined in claim 1 in which the amplification orifice is fixed.
 15. Amethod as defined in claim 1 wherein the hydraulic pump is a variabledisplacement hydraulic pump having a pump margin pressure and thecontroller is configured to vary a margin pressure of the main orificerelative to the pump margin pressure in response to the output signal.16. A method as defined in claim 1 in which the hydraulic pump is afixed displacement hydraulic pump, having a pump margin pressure definedby a bypass regulator valve and the controller is configured to vary amargin pressure of the main orifice relative to the pump margin pressurein response to the output signal.
 17. A method as defined in claim 15wherein the controller is configured to increase the margin pressure ofthe main orifice relative to the pump margin pressure in response to theoutput signal.
 18. A hydraulic system including a hydraulic pump havinga pump outlet, a main orifice having a main orifice inlet in fluidcommunication with the pump outlet and a main orifice outlet forsupplying pressurized fluid to a service, a flow orifice having a floworifice inlet for sensing a pressure representative of a pressure at thepump outlet and a flow orifice outlet, an amplification orifice havingan amplification orifice inlet in fluid communication with the floworifice outlet and an amplification orifice outlet for sensing apressure representative of a pressure at a service, means for generatingan output signal representative of a fluid pressure between the floworifice outlet and the amplification orifice inlet, and a pumpcontroller for controlling the hydraulic pump in response to the outputsignal, the flow orifice defining a flow orifice cross section area andthe amplification orifice defining an amplification orifice crosssection area, the flow orifice cross section area and the amplificationorifice cross section area defining a ratio, in which the main orificeis variable and the ratio is variable, the main orifice defining a mainorifice margin pressure, the system being configured to vary the mainorifice margin pressure by varying the ratio.
 19. A hydraulic system asdefined in claim 18 wherein the pump defines a pump margin pressure andthe system is configured to vary the main orifice margin pressurerelative to the pump margin pressure.
 20. A hydraulic system as definedin claim 19 wherein the main orifice margin pressure is greater than thepump margin pressure.